Composites News

Helping to Standardize High-Rate Testing of Composites

Instron® has joined a new international group that is seeking to develop a best practice guide and test standards specifically for testing composites at high-strain rates.

As the automotive industry seeks ever-more-urgently to embrace composites, there is an increasing demand for testing composite material behavior at high-strain rates. The need for detailed data to inform crash simulation models first drove a renewed demand for equipment over the last 3 years, and now there is a need for international standardization in methodologies and data handling. The group’s aim is to facilitate generation and exchange of reliable and comparable test data in this highly challenging area.

The working group has been coordinated by the University of Dayton Research Institute and currently composes about 20 organizations including major automotive manufacturers, composite materials producers, test houses, and research institutes. As a world leader in high-rate servohydraulic testing systems, the dynamic systems team at Instron are very pleased to share their expertise with this initiative that will make a tangible difference to the industry. Similarly, Instron CEAST will be contributing to work on drop-weight based techniques for high rate testing.

The working group is looking for more European contributors especially, but we would strongly encourage all our customers with expertise in this area to join us in supporting the project. Please feel free to contact Instron applications specialist, Dr. Peter Bailey, if you would like to know more.

Partnering with the University of Manchester

A new Instron 8806 2.5 MN testing machine has been installed at the National Composite Certification and Evaluation Facility (NCCEF) within the Aerospace Research Institute (ARI) at University of Manchester in the UK. NCCEF has been ear-marked to advance and refine fundamental polymer composite testing science in the UK. The machine is the first installation in the new national large-scale composite research facility and it will be dedicated to fundamental research on composite behavior.

The facility will consist of the 2.5 MN testing machine capable of testing 2 m long substructures, plus a wide range of non-destructive testing equipment used to monitor the behavior of composite materials during tests.

The first project at this new facility is a two-year, £5.9 m research collaboration of NCCEF with Airbus UK, the National Composites Centre and Spirit Aerosystems. It is funded through the UK Technology Strategy Board (TSB) and the Aerospace Technology Institute (ATI) (Concept Integration – ‘Wings of the Future’, 2014-2016).

Key issues are the better understanding of subsurface damage formation and the applicability of coupon-derived material data to the design of extended substructures at full scale. In the long term, the new facility will allow NCCEF to supply critical research and development support to multiple strategic aerospace primes and first-tier suppliers in UK aerospace and automotive sectors.

Tensile-Impact Testing

Today, composite materials are broadly used to create parts that require a high strength, yet are low weight. The most critical structural failure mode for this kind of materials is mainly due to impact damage. The impact, most of the time at low-energy, is able to produce delamination and/or fibers breakage in the deep layers of the composite without any significant event on the surface. As a consequence and without notice, a delamination can propagate over time and suddenly appear with catastrophic consequences. For this reason, it is essential to understand the behavior of the fibers and matrix under impact conditions in order to optimize their characteristics and the sequence of stratification.

Although Charpy and Izod impact testing techniques are still occasionally utilized for composite materials, emphasis has shifted during the last few years to other techniques. This is due to the possibility that a very complex stress state can be induced into the composite during Charpy or Izod testing, resulting in a wide variety of failure modes. For instance, it is not uncommon to see tension, compression, and shear exhibited in the same specimen during a test. This makes it difficult to compare the impact performance of composite materials.

Of the large number of possible directions of future research in impact behavior, the tensile-impact techniques that achieve high-loading rates appear to be particularly promising.

Very recently, Instron presented the newly developed set of accessories for tensile-impact of composites to be used in combination with a drop tower impact machine. This particular test setup allows the operator to measure the amount of force / energy required to break a composite specimen under a pure tensile load at high speed (high tensile rates). Additionally, the drop tower allows the operator to explore impact velocities ranging from 0.7 m/s (relatively low rate) up to 15 m/s (high tensile rates).

The setup consisted of a bar-shaped specimen mounted in a fixed grip that was connected to a sturdy fixture. The configuration is completed by a tup and the force sensor, capable of measuring loads up to 30 kN. The tup impacted the crosshead that was attached to the other end of the specimen.

During impact, the specimen was subjected to a pure axial tensile load while the force was constantly measured. The acquired data, sampled with frequencies up to 4 MHz, is then transmitted from the data acquisition system directly to the software. It allowed for plotting the forces and energies absorbed during the various stages of the test: by elastic deformation, to failure of the fibers and the matrix, as well as to the propagation of the fracture along the specimen or to the delamination occurring. This data, appropriately analyzed, shows the full characterization of the impact properties of the composite material from low to high loading rates.

Figure 2 shows the force acquired by the strain-gauge sensor as a function of the impact time during a tensile-impact test on a carbon fibers composite at a velocity of 5 m/s.

Conference Update: ICEM 16

Our colleagues attended the 16th International Conference on Experimental Mechanics (ICEM 16) held at the University of Cambridge. The conference was very lively with a truly international attendance. During the conference, we exhibited an ElectroPuls™ test instrument and an Electromechanical machine equipped with DIC Software.

In addition to exhibiting our products, our very own Dr. Peter Bailey presented a paper entitled the "Effects of Self-Heating and Specimen Design in Composites Fatigue Testing". Lastly, we were very pleased to be able to provide a testing system for the BSSM Measurement lecture "Materials at Their Limit", given by Professor John Dear of Imperial College (London).